The present invention relates to a polishing apparatus, more precisely relates to a polishing apparatus, which is capable of simultaneously polishing both side faces of a work piece held by a carrier engaging with a sun gear and an internal gear provided between an upper polishing plate and a lower polishing plate, with the polishing plates.
A conventional apparatus for simultaneously polishing both side faces of work pieces, e.g., silicon wafers, crystal plates, is shown in FIG. 6. In the apparatus shown in FIG. 6, carriers 208 driven by an internal gear 204 and a sun gear 206 are provided between a lower polishing plate 200 and an upper polishing plate 202 rotating in prescribed directions. Each of the carriers 208 has a plurality of through-holes for holding the work pieces, and the both side faces of the work pieces are simultaneously polished by the polishing plates 200 and 202.
The lower polishing plate 200 is mounted on a lower plate holder 209 and rotated together with the lower plate holder 209. The lower plate holder 209 is rotatably mounted on a base 210 with a bearing 212 and rotated by torque transmitted from an electric motor 214 via a gear 216 and a cylindrical shaft 217.
The upper polishing plate 202 is rotated by torque transmitted from an electric motor 224 via a gear 218 and a shaft 219. The internal gear 204 is rotated by torque transmitted from an electric motor 226 via a gear 220 and a cylindrical shaft 221. Further, the sun gear 206 is rotated by torque transmitted from an electric motor 228 via a gear 222 and a cylindrical shaft 223.
As shown in FIG. 7, a plurality of the carriers 208 are mounted on the lower polishing plate 200, and a plurality of through-holes 230, in each of which the work piece will be held, are formed in each carrier 208.
Further, gear teeth, which engage with the internal gear 204 and the sun gear 206, are formed on an outer circumferential face of each carrier 208. Therefore, when the gears 204 and 206 rotate, the carriers 208 are spun about their own axes and moved around the sun gear 206 by a rotational speed difference between the gears 204 and 206.
The work pieces are respectively held in the through-holes 230 of the carriers 208, the carriers 208 are provided between the polishing plates 200 and 202, and the polishing plates 200 and 202 and the gears 204 and 206 are rotated at prescribed rotational speeds. With this action, the work pieces are spun and moved round the sun gear 206 together with the carriers 208, and their both side faces are simultaneously polished by the polishing plates 200 and 202.
These days, work pieces become thinner and thinner. Thus, the carriers 208 and/or the work pieces held by the carriers 208 are easily damaged or broken while polishing.
One of reasons causing the damage or break is vibrations. In the polishing apparatus shown in FIG. 6, torques of the motors 214, 224, 226 and 228 are transmitted to the shafts via the gears 216, 218, 220 and 222, reduction gear units, etc. Namely, vibrations are easily generated between the gears, and the vibrations are apt to damage the work pieces held by the carriers 208.
Further, when the polishing operation starts, it is difficult to synchronize the motors 214, 224, 226 and 228 due to backlashes of the gears 216, 218, 220 and 222, and the reduction units. Overloads are applied to the carriers 208, so that the carriers 208 may be damaged or the work pieces may jump out.
To reduce the vibrations and backlashes of the gears, another polishing apparatus shown in FIG. 8 was invented (see Japanese Patent Gazette No. 2000-127033).
In the apparatus shown in FIG. 8, an upper end of a rotary shaft 302, which is directly driven by a direct drive (DD) motor 300, is connected to a polishing plate 304. A polishing cloth 306 is adhered on a surface of the polishing plate 304. A work piece 312 is adhered on an adhering block 310 of a carrier head 308. The carrier head 308 is provided to a lower end of a rotary shaft 316, which is directly rotated by a DD motor 314. A side face of the work piece 312 is pressed onto a polishing face of the polishing cloth 306 with a prescribed pressing force, so that the side face of the work piece 312 can be polished.
In the apparatus shown in FIG. 8, the polishing plate 304 and the carrier head 308 are directly rotated by the DD motors 300 and 314 without transmission means. Therefore, damaging and breaking the work piece 312 caused by vibrations and backlashes of gears can be prevented. Even thin work pieces can be properly polished. Note that, the polishing apparatus cannot simultaneously polish the both side faces of the work piece 312.
The apparatus shown in FIG. 8 has two rotary shafts 302 and 316 for rotating the polishing plate 304 and the carrier head 308. On the other hand, the apparatus shown in FIG. 6, which is capable of simultaneously polishing the both side faces of the work pieces, has four rotary shafts 217, 219, 221 and 223 for rotating the polishing plates 200 and 202 and the gears 204 and 206. Generally, DD motors are larger than ordinary electric motors. So if three or more rotary shafts are respectively driven by DD motors, the polishing apparatus must be large.